The present invention relates to a fiber-treatment agent. Fiber-treatment agents based on organopolysiloxane containing groups with the formula --CH.sub.2 CH.sub.2 CH.sub.2 NHCH.sub.2 CH.sub.2 NH.sub.2 have been used in order to impart lubricity to fibrous materials composed of natural fibers such as cotton, flax, silk, wool, angora, and mohair; regenerated fibers such as rayon and Bemberg; semisynthetic fibers such as acetate; and synthetic fibers such as polyesters, polyamides, polyacrylonitriles, polyvinyl chlorides, Vinylon, polyethylenes, polypropylenes, and Spandex (refer to Japanese Patent Publication Number 57-43673 (43,673/82)). However, fibers treated with such an organopolysiloxane are subject to yellowing due to a spontaneous oxidation occurring with time. Moreover, when a continuous lubrication using rollers is carried out from a bath containing such an organopolysiloxane lubricant, moisture and carbon dioxide are absorbed from the atmosphere, a white turbidity appears in the bath, and the precipitation of a gel occurs. In addition, when such an organopolysiloxane is used for high-temperature oiling or lubrication as in the treatment of carbon fiber, for example, polyacrylonitrile-based carbon fiber, the organopolysiloxane is degraded to a gum, which sticks on the rollers, etc. This has the unfortunate effect of causing the fiber to snap.